Global implementation of Automatic Dependent Surveillance (ADS) is underway and promises to replace conventional radar surveillance (e.g., Secondary Surveillance Radar, or SSR) within the next 10 to 20 years. Switching to ADS from radar techniques represents a fundamental shift in the techniques and philosophy of aircraft tracking.
Aircraft tracking relying on multilateration techniques are well established in the air traffic control industry with several companies supplying systems including Rannoch Corporation (www.rannoch.com), Sensis Corporation (www.sensis.com), and ERA (www.era.cz). These systems rely on the time stamping of transponder replies, either at remote sensor locations, or at a central time reference. The aircraft or vehicle location is then computed from the time difference of arrival of the transponder signal, as the transponder signal is received at multiple sensor locations. For a description of time difference of arrival multilateration, please refer to: http://en.wikipedia.org/wiki/Multilateration.
A system for elliptical surveillance uses one or more receiving elements, one or more transmitting elements, and a central workstation. The transmitting elements and receiving elements are time-synchronized to a common precision time reference. The antennas for the transmitting elements are separated in distance from the antennas from the receiving elements and are located at known positions. At a given time, one of the transmitting elements transmits an interrogation signal to one or more targets. The target(s) respond to the interrogation with a reply transmission. The reply transmission is received by one or more receiving elements and each target's ellipse of position with respect to each receiving element is calculated by the central workstation using the interrogator element time of interrogation measurement and each receiving element's time of arrival measurement for the corresponding reply transmission. The central workstation fuses the elliptical lines of position for each receiving element to compute target positions or augment passive surveillance position. At a scheduled time, the one or more transmitting elements transmit a reference signal to the receiving elements(s). The central workstation uses each of the receiving elements time stamped signals to perform integrity monitoring.
There are a variety of systems that emit pulses and other radio signals on board most aircraft including transponders, DME, TACAN, ADS-B, UAT, and various data link systems. Systems exist today, to multilaterate position using a combination of fixed ground based receiver systems, such as the PSS VERA E (http://www.era.cz/en/pss-vera-e.shtml).
Other systems that embrace the ADS-B concept of position self-reporting include Iridium, FANS (Future Air Navigation System), ACARS and CPDLC, as described below:                Iridium satellite is used for voice and data, including aircraft position self-reports. (http://www.iridium.com/corp/iri_corp-news.asp?newsid=111).        The International Civil Aviation Organization (ICAO), the International Air Transport Association (IATA), and other air traffic service providers developed the concept of FANS to transition the current air traffic control system to an advanced air traffic management system utilizing satellite based communications, navigation and surveillance for all regions of the world for the twenty-first century. For more information on FANS refer to: www.simlabs.arc.nasa.gov/library/technical_papers/aiaa96.html and http://www.icao.int/icao/en/ro/apac/2003/cnsatmgmtf/CnsAtmGM_TFReport.pdf#search=‘FANS %20ICAO        The Aircraft Addressing and Reporting System (ACARS) is a widely used commercial service that includes aircraft self-position reports. A good description of ACARS is available from: www.ARINC.com.        Controller Pilot Data Link (CPDLC) and VDLM2 offer higher data rates for aircraft messaging that include self position reports. An excellent description and comparison of ACARS, CPDLC, and VDLM2 was presented by John Burke of ARINC at the ICNS conference in Annapolis in May, 2003. (http://spacecom.grc.nasa.gov/icnsconf/docs/2003/04_B1/B1-04-Burks.pdf#search=‘cpdlc’).        
In addition to aircraft, vehicles and ships also carry systems that provide self-reporting over a data link, for example the IBM VTS/2000 system (http://www-304.ibm.com/jct09002c/gsdod/solutiondetails.do ?solution=938&expand=true&lc=en).
As the transportation industry moves toward real-time global tracking and identification of all forms of transport, there is recognition that relying on self-reporting is vulnerable to errors, or intentional spoofing, and other security issues. This is probably most acute in aviation following the events of 9/11 and the potential damage that can result from aircraft accidents. In aviation, the need for a potential back up or validation is the subject of debate at the FAA (www.faa.gov), the Next Generation of Air Traffic Systems (NGATS) and the Joint Planning and Development Office (JPDO). For more information on these organizations and aviation initiatives please visit (www.jpdo.aero).
Although, as of early 2006, there is no overall plan for a particular back-up or validation technology, the use of combinations of sensor systems has been discussed by FAA, NGATS, and JPDO. Sensor systems discussed include mixes of conventional radar systems, multilateration, passive tracking, and other techniques.
A discussion of various potential back-up methods is presented in U.S. Department of Transportation Federal Aviation Administration Safe Flight 21—Future Surveillance Broadcast Services Ground Station (BSGS) Specification Using the Universal Access Transceiver (UAT) and 1090 MHz Extended Squitter (1090ES) ADS-B Data Links, Draft, Version 2.0, 12 Dec. 2005.
There are many issues involved in selecting an appropriate back up technology including:                Practicality        Performance        Independence        Cost        Business case        
The business case for ADS-B is to forgo the use of older more expensive radar technologies, in favor of a more flexible ADS self-reporting technology. It is has been difficult thus far to achieve general consensus on an optimal approach using ADS and some form of back-up. The use of encryption or secure transponders to prevent spoofing has been discussed, but there are many cost equipage issues, avionics standardization, and the ability to spoof may be reduced but will not be eliminated. (See, e.g., Viggiano, U.S. patent application Ser. No. 10/285,070, Publication No. 20040086121, entitled “Secure Automatic Dependent Surveillance,” incorporated herein by reference).
Other possible methods to confirm the authenticity of ADS-B include comparison and correlation between ADS-B and TCAS, such as that described in U.S. patent application Ser. No. 10/923,433, Publication No. 20050231422, entitle “Systems and Methods for Correlation in an Air Traffic Control System of Interrogation-Based Target Positional Data and GPS-Based Intruder Positional Data,” incorporated herein by reference. However, although probably a good approach for aircraft to aircraft surveillance and target verification, it is not practical for an ATC-wide application, and would still rely on some form of self-reporting. Also, TCAS is not designed to provide sufficient lateral accuracy for ATC surveillance.